BI-METAL SCREW OR BOLT ADAPTED TO FORM ELECTRIC CONTACT ELEMENTS FOR TERMINAL STRIPS OR THE LIKE

DESCRIPTION

The present invention relates to a bi-metal screw or bolt adapted to form electric contact elements for terminal strips or the like.

At one time, these contact screws or "bolts", comprising a stem, generally threaded, and a contact head, were made entirely of copper or copper alloys. However, over time the cost of these elements made of copper or copper alloys proved to be prohibitive, and therefore the stem and also part of the head of these elements has been replaced with less costly metals, and in particular with a ferrous material, such as steel, producing only that part of the head in which electric contact takes place physically in copper or copper alloy.

However, the problem that occurs in the manufacture of a steel-copper bi-metal bolt of this kind lies in the production of an intimate contact between the surfaces of these two metals, without areas of discontinuity in which electric arcs could form, and in making these surfaces as extensive as possible to avoid resistance areas in the passage between a conductor element with high electric conductivity to another conductor element with lower electric conductivity.

There are known in the art various bi-metal bolts tending to solve the problem of obtaining a contact bolt for electric terminals strips or the like, which guarantees a reliable and safe electric contact, while decreasing the costs of conventional copper or copper alloy terminals.

The patent GB 1284954 (A) discloses a method for manufacturing electric contact bolts according to which there is formed in the centre of a head of a steel bolt a recess provided with undercut parts, and there is inserted into this recess a copper plate on which a pressure is exerted such as to cause plastic deformation thereof, so as to completely fill the recess of the head of the bolt.

A similar method is described in the patent US No. 4821412, according to which a blind hole is formed at one end of the head of a contact base element made of ferrous metal, and a block made of copper, or another good conductor metal, is inserted into this hole, and subsequently a pressure is exerted on this head so that these two elements undergo plastic deformation until they are joined in intimate contact.

A similar system is described in the patent application EP 1995745 A1.

However, none of these prior art bi-metal bolts have proven to be capable of completely satisfying the requirements of users, as on the one hand they frequently give rise to points of discontinuity between the two metals, which can easily lead to the formation of electric arcs, and on the other hand the contact surface between base metal and conductor metal is relatively limited.

Therefore, the object of the present invention is to produce an electric contact screw or bolt comprising an element made of non-precious metal, such as steel, comprising a stem and a head, there being set into the head with intimate contact an element made of a good electric conductor metal, such as copper or a copper alloy. However, unlike the teachings of the state of the art, the head of the screw according to the present invention has no central recess, but instead has a peripheral annular groove with directrices that converge with an angle of the order of 10 to 15° toward the central axis of the stem of the screw, the contact element made of copper or copper alloy consisting of a plate provided with an annular collar substantially the same height as the depth of said groove, and having a thickness corresponding substantially to the width of said groove and a diameter corresponding to the inlet diameter of said groove, so that when the annular collar of the contact element is forced into said groove it undergoes plastic deformation, filling the same groove completely and becoming irremovably crimped thereto. Moreover, importantly, the contact surface between the copper insert and the steel head is much more extensive than that of prior art bi-metal terminals, which represents a considerable technical advantage of the screw according to the present invention.

Further objects, features and advantages of the screw or bolt according to the invention will be more apparent from the following description of a preferred embodiment of the same, provided with reference to the accompanying drawing, wherein:

Fig. 1 is a radial longitudinal sectional view of a screw according to the present invention, in the head of which there has been formed an annular groove with directrices that converge with an angle of the order of 10 to 15° toward the central axis of the stem of the screw;

Fig. 2 is a top plan view of the screw of Fig. 1 ;

Fig. 3 is a radial longitudinal sectional view of an insert made of a conductor metal to be inserted into the screw of Fig. 1 , and

Fig. 4 shows the screw of Fig. 1 as it appears after pressurized insertion - with plastic deformation - of the conductor metal insert of Fig. 3 into the annular groove of the head thereof.

With reference to the drawings, and in particular with reference to Figs. 1 and 2 thereof, the screw illustrated comprises a threaded stem 1 surmounted by a head 2. The reference numeral 3 indicates a peripheral annular cut with directrices that converge conically towards the axis of the screw 1 with an inclination of the order of 10 to 15° with respect to the central axis of the stem 1 of the screw.

In Fig. 3, the reference numeral 4 illustrates in a radial longitudinal section an insert made of a good conductor metal, such as copper or a copper alloy, intended to be inserted into the annular cut 3 of the head 2 of the screw 1 so as to form the electric contact element. This insert consists of a plate 5 provided peripherally with a cylindrical annular collar 6 substantially the same height as the depth of the groove 3, and having a thickness corresponding substantially to the width of said groove 3 and a diameter corresponding to the inlet diameter of said groove 3, so that when the annular collar 6 of the contact element 4 is forced into the groove 3, it is plastically deformed adapting to the conicity of the groove 3 filling the same groove 3 completely, and being irremovably crimped thereto. Moreover, importantly, the contact surface between the good conductor metal insert 4 and the steel head is much more extensive than that of prior art bi-metal terminals, as it comprises the two surfaces, inner and outer, of the annular collar 6 and the bottom surface of the plate 5, which represents a considerable technical advantage of the screw according to the present invention.

Naturally, the present invention is not limited to the form of embodiment of the electric contact screw illustrated and described. Therefore, by way of example, the head of the screw can have any peripheral shaping suitable for the purpose, and, by way of example, can be shaped like a square or hexagonal bolt, the plate 4 can be provided on the contact surface with concentric projections 7, as illustrated, to guarantee a better and safer contact. This plate can be made of copper or of a copper alloy, or of another good electric conductor metal.

Therefore, all those variations and modifications falling within the wider scope of the accompanying claims fall within the scope of the present invention.